1. Field of the Invention
The present invention relates to electrical energy harvesting systems and, more specifically, to an electrical energy harvesting system that is rectifier free.
2. Description of the Related Art
Microscale and nanoscale integrated devices are used in a variety of applications, including in biomedical implants and sensors. Many such devices are powered by electrical energy. Conventional power sources, such as electrochemical batteries, are often used to power such integrated devices. However, in certain applications, such as biomedical implants, replacing or recharging batteries may be difficult and costly. One approach to recharging such batteries is to harvest electrical energy from piezoelectric structures subjected to ambient motion (such as muscle flexing in the case of biomedical implants or vibration in the case of environmental sensors, etc.). Typical piezoelectric structures include cantilevers made from a piezoelectric material, such as certain crystals, organic materials and ceramic structures, generate electrical charge when subject to mechanical force. For example, a zinc oxide crystal will generate a charge imbalance when subjected to a bending force. Other piezoelectric structures generate charge when subjected to stress or strain.
Given that many piezoelectric structures are subject to periodic movements that have both a positive cycle and a negative cycle (such as a vibrating cantilever), the electricity generated by a piezoelectric structure is typically in the alternating current form. However, most batteries require direct current to be recharged. Therefore, recharging a battery from a piezoelectric structure usually requires a device to convert the alternating current to direct current.
A typical microscale alternating current to direct current converter includes a diode bride rectifier. Such a rectifier employs an arrangement of diodes that channels current during both the positive cycle and the negative cycle onto an output in the form of a series of positive half-waves. In such a rectifier, current from the piezoelectric structure usually must pass through two diodes and each diode typically has a voltage drop of between 0.2 V to 0.7 V (depending on the semiconductor technology employed). This voltage drop can be greater than the voltage output from the piezoelectric structure and, therefore, usually requires that a voltage boost be applied to the output from the piezoelectric structure so that the voltage being applied to the battery is greater than the voltage of the battery, otherwise no current would flow into the battery. Such a system tends to waste a considerable amount of energy.
The problem is rectifying unpredictably small ac signals (which are prevalent in small volumes and with weak vibrations whose peak voltages fall below the rectified output level targeted, requires low-loss, no-threshold rectifiers. Quasi-lossless LC energy-transfer networks that precede or follow the rectifier can extract all the energy stored in the piezoelectric capacitance and therefore overcome the basic threshold-voltage limitation, except the rectifier and its controller's headroom and quiescent current nonetheless limit the input voltage range of the system and dissipate power.
Miniaturized mobile electronic systems, such as biomedical drug-delivery implants, acceleration-monitoring and pressure-monitoring sensors, and micro-sensor nodes in wireless networks, have so little space for energy storage that they suffer from short operational lives. Unfortunately, replacing easily exhaustible onboard batteries is prohibitive because the systems often conduct in situ measurements in unreachable places and operate in concert with numerous other devices, where the personnel and logistical costs of maintaining all batteries charged are unacceptably high. Harvesting energy, however, from light, heat, radiation, and motion is an attractive, though not easy alternative for replenishing small batteries and capacitors.
Although the application ultimately determines what energy source suits best, harvesting kinetic energy is promising because motion and vibrations are abundant and produce moderate power levels. For context, solar light generates more power, but only when exposed directly to the sun, and power densities derived from indoor lighting, thermal gradients, and radio-frequency (RF) waves fall well below their kinetic counterparts, although not all motion-based transducers perform equally well. Piezoelectric devices, in fact, when constrained to small platforms, generate more power than variable (electrostatic) capacitors and moving (electromagnetic) coils.
When considering a piezoelectric source, the internal charge configuration of the material changes (much like an ac current source) to generate an alternating voltage across the equivalent capacitance that its opposing surfaces present. The harvester circuit must therefore extract energy from the changing voltages of the piezoelectric capacitor and deposit charge into an energy-storage device. Because harvested power is low and uncorrelated to the load, a small battery or capacitor serves as the reservoir from which electrical functions in the system draw power on demand.
Conventional approaches first rectify the incoming ac voltage with a diode bridge. Some techniques reduce the voltage (and therefore power losses) across the pn-junction diodes by using MOS switches and driving them with a comparator that senses and ensures only small positive terminal voltages allow the switches to close. Unfortunately, input voltages must nonetheless exceed their rectified outputs for the MOS switches to conduct, which means rectifiers place a threshold limit on the mechanical input. In other words, rectified harvesters only harvest energy above a minimum input level, so they cannot extract all the energy the piezoelectric material offers. Although some systems extract more energy from the environment by boosting the transducer's electrical damping force with higher (LC-induced) piezoelectric voltages, the subsequent rectifier still suffers from a threshold minimum below which the harvester cannot harness energy. Furthermore, drawing maximum power requires an optimal rectified output voltage, so some approaches, at the cost of power, employ a correcting feedback loop that senses the harvester's output current to set the optimal rectified level.
Therefore, there is a need for an alternating current to direct current converter that does not incur the losses associated with a bridge rectifier.